1. Field of Art
The present invention relates to semicrystalline polymers and particularly to semicrystalline polymers such as biaxially oriented polyester film whose surface has been treated to reduce its static and kinetic coefficients of friction.
2. Background of the Art
A problem that has often occurred in the handling, manipulating or processing of polymeric material is the relatively large coefficient of friction that exists between film surfaces. Biaxially oriented poly(ethyleneterephthalate) is a film material that is used extensively in commerce and manufacturing and is well recognized as presenting a problem with respect to an excessive coefficient of friction between adjacent sheets or plies. In many uses, such as in the handling and processing of imageable materials with poly(ethylene terephthalate) (PET) substrates, it is not desirable to use lubricants (either dry or wet) between the films. The drag between adjacent films can cause problems of multiple feeding of sheets, partial feeding of sheets, and skewing of sheets during automatic or even manual processing.
The problem is most readily manifest in the winding of PET film which contains neither so-called internal slip agents nor lubricating additives. The self-adhesion of adjacent plies is such that great force is required to separate those plies in an unwinding operation, and that force is frequently sufficient, particularly in the case of thin gauge films, to cause an unacceptable number of film breaks. Moreover, it has been established that to avoid troublesome film handling and processing the static and sliding coefficients of friction would be desirably less than 0.8, preferably less than 0.6.
One standard solution to this problem of the coefficient of friction is to incorporate particles in the film, using so-called internal slip agents. The presence of these particles (usually in the range of 0.1 to 3% by weight of the polymer), roughens the film surface and reduces the static coefficient of friction into the range of about 0.3 to 0.5. Although this technique has been used for many years, there are significant drawbacks to this practice. Not only are film costs increased by this procedure, but reduced film transparency which results from light scattering effects, and the increased roughness caused by multiple protrusions of the particulates in the surface of the film are frequently detrimental.
The effects of actinic radiation on the degradation of polymer surfaces have been studied for many years. Prior to about 1970, this work was done with low intensity photolamps at wavelengths greater than 220 nanometers (nm). Numerous papers are available in the literature, typical of which are Day and Wiles, Journal of Applied Polymer Science, 16, 175 (1972), and Blais, Day and Wiles, Journal of Applied Polymer Science, 17 p. 1895 (1973).
Between 1970 and 1980 the effects on polymer surfaces of ultra-violet (UV) lamps with wavelengths less than 220 nm were studied for lithography and surface modification purposes. Such studies are exemplified by Mimura et al., Japanese Journal of Applied Physics, 17, 541 (1978). This work illustrates that long exposure times and high energies are required to cause photo-etching when UV lamps are used. U.S. Pat. No. 3,978,341 (Hoell) teaches an apparatus for exposing polymeric contact lenses to a spark discharge producing 83 nm to 133.5 nm UV radiation to improve the wettability and adhesiveness of the lenses.
In 1975 the excimer laser was discovered. An excimer laser is an excited dimer laser where two normally non-reactive gases (for example Krypton, Kr, and Fluorine, F.sub.2) are exposed to an electrical discharge. One of the gases (Kr) is energized into an excited state (Kr*) in which it can combine with the other gas (F.sub.2) to form an excited compound (KrF*). This compound gives off a photon and drops to an unexcited state which, being unstable, immediately disassociates to the original gases (Kr and F.sub.2), and the process is repeated. The released photons are the laser output. The uniqueness of the excimer laser lies in its high efficiency in producing short wavelength (UV) light and its short pulse widths. These attributes make the excimer laser useful for industrial applications. Kawamura et al., Applied Physics Letters, 40 374 (1982) reported the use of a KrF excimer laser at 248 nm wavelength to photo-etch polymethyl methacrylate (PMMA), a polymer used in preparing photolithography resists for semiconductor fabrication.
U.S. Pat. No. 4,414,059 (Blum, Brown and Srinivasan) disclosed a technique for the manufacture of microelectronic devices utilizing ablative photodecomposition of lithography resist amorphous polymers at wavelengths less than 220 nm and power densities sufficient to cause polymer chain fragmentation and immediate escape of the fragmented portions. The photodecomposition leaves an etched surface. The authors found that using an ArF excimer laser at 193 nm with a 12 nanosecond pulse width, a threshold for ablatively photo decomposing poly(methylmethacrylate) resist material occurs at a fluence of 10-12 mJ/cm.sup.2 /pulse. It is stated that large amounts of energy, greater than the threshold amount, must be applied before ablation will occur. The energy used must be (1) sufficiently great and (2) applied in a very short amount of time to produce ablative photodecomposition.
U.S. Pat. No. 4,417,948 (Mayne-Banton and Srinivasan) and a related publication, Srinivasan and Leigh, Journal American Chemical Society, 104, 6784 (1982) teach a method of UV photo etching poly(ethylene terephthalate) (PET). In these publications the authors indicate the mechanism of photo etching to be one of chain scission or bond breaking of surface polymer molecules by the high energy UV. Bond breaking continues in the presence of irradiation and the smaller units continue to absorb radiation and break into still smaller units until the end products vaporize and carry away any excess photon energy. This process results in small molecules being ablated away, and various gases being evolved. The remaining surface material comprises molecular mixtures of molecules of low molecular weight (e.g., oligomers). Examining the PET repeating unit and the author's claim of bond scission, it is believed that the following occurs: ##STR1## Indeed, in the Journal of the American Chemical Society article, the authors analyze for benzene and start detecting it at about 20 mJ/cm.sup.2 /pulse at 193 nm. The authors also indicate that the photo etch process is accelerated in the presence of oxygen which seals the ends of the broken chain's fragments and prevents recombination of these fragments.
Srinivasan, Journal of the Vacuum Society, B1, 923 (1983) reports the results of ablative photodecomposition of organic polymers through a 0.048 cm diameter mask and states that a threshold exists for the onset of ablation and, for PMMA, that the threshold is 10 mJ/cm.sup.2 /pulse. He then goes on to state that one pulse at 16 mJ/cm.sup.2 /pulse gave an etch mark on PMMA while 50 pulses at 4 mJ/cm:/pulse left no detectable etch marks. For PET and polyimide, the threshold for ablation began at about 30 mJ/cm.sup.2 /pulse. However, for a satisfactor 100 to 350 mJ/cm.sup.2 /pulse.
In Srinivasan and Lazare, Polymer, 26, 1297 (1985) Conference Issue, the authors report the photo etching of 6.times.12 mm samples of PET, PMMA and polyimide polymers with both continuous radiation at 185 nm from UV lamps and pulsed radiation at 193 nm from an excimer laser. The use of continuous low energy UV lamps caused photo oxidation of the polymer surface with a resultant increased oxygen-to-carbon ratio (O/C. ratio) as determined by x-ray photoelectron spectroscopy (XPS) equipment, while the use of a pulsed high energy excimer laser, which produces chain scission in and ablation of the polymer surface, resulted in a lower O/C. ratio as determined by XPS. The authors then go on to say "It may be pointed out that ablative photo decomposition is not exactly a method for the modification of a polymer surface at an atomic level since it totally eliminates the atoms at the surface and creates a fresh surface."
U.S. Pat. No. 3,607,354 discloses the use of highly active hydroxybenzene solvents to deluster the surface of an oriented polyethylene terephthalate film. The solvent acts to dissolve and swell the poly(ethylene terephthalate) and remains in the surface layer. The chemical composition of the surface layer is different from that of the bulk polymer because of the presence of the very active solvents at the time that the film is coated. The delustering may in fact indicate that crystalline spherulites of such a large size are produced that they scatter light.
U.S. Pat. No. 4,568,632 (Blum, Holloway and Srinivasan) claims a method for photo etching polyimides. The process described uses a pulsed excimer laser at 193 nm. The stated incident energy required for photo ablation is much higher for polyimide than for PET. The values for the laser fluence threshold of PET was reported as about 30 mJ/cm.sup.2 /pulse while for polyimide it was reported as about 50 mJ/cm.sup.2 /pulse. An operative level was noted as about 50-100 mJ/cm.sup.2 /pulse for PET and 100-300 mJ/cm.sup.2 /pulse for polyimide. The etch rate found for PET was 1000 Angstroms/per pulse for a fluence of 100-300 mJ/cm.sup.2 /pulse and for the polyimide was 750 Angstroms for 350 mJ/cm.sup.2 /pulse.
Lazare and Srinivasan, Journal Physical Chemistry, 90, 2124 (1986) report on the study of surface properties of PET which have been modified by either pulsed UV laser radiation or continuous UV lamp radiation. The authors report on the high fluence ablation of PET by pulsed excimer laser radiation as follows: (1) the PET irradiated surface is a layer of low molecular weight material, (2) the surface has a rough, chemically homogeneous texture, (3) the surface has a high chemical functionality characteristic of oligomers, and (4) the surface could be removed by washing in acetone. Since extremely low molecular weight fragments (oligomers) of PET are soluble in acetone, the authors assert this removal of the treated surface is indicative of the presence of low molecular weight material on the surface. The authors also report that the low intensity UV lamp treated PET surfaces would not wash off with acetone. This later article reports thresholds for ablation of PET at about 30-40 mJ/cm.sup.2 /pulse.
Japanese Patent Publications JA 59-82380, JA 59-101937 and JA 59-101938 (Kitamura, Veno and Nomura) describe the treatment of various polymers with many pulses from moderately high energy lasers for the purpose of increasing adhesion and forming a barrier layer to prevent plasticizer migration from within certain polymers
Bishop and Dyer, Applied Physics Letters, 47, 1229 (1985) extended the photoablation etching work of others to actually cutting through or slitting the polymer film by increasing the energy density of the laser beam by appropriate focusing techniques.
The authors of the above references were studying the photodecomposition or photoablation process of UV radiation on polymer surfaces, without regard to whether the polymer was semi-crystalline or amorphous. The present invention does not produce substantial photodecomposition and little or no photoablation, and is concerned only with the pulsed UV radiation of semicrystalline polymers using exposure intensities in an energy regime different from those used in the prior art.
"Polymer Interface and Adhesion", Souheng Wu, Published by Marcel Dekker, Inc., N.Y. and Basel, Chapter 5, page 206 indicates that when a polymer melt cools and solidifies, an amorphous surface is usually formed, although its bulk phase may be semicrystalline. This is at least in part a result of fractions not accommodated in the crystalline structure being rejected to the surface. This amorphous surface is not recrystallizable because of the presence of the fractions and is believed to be extremely thin, corresponding to only a few layers of molecules, and is of the order of no more than 2 or 3 nm, and is generally less than 2 nm in thickness.
U.K Pat. No. 1,579,002 discloses vacuum glow discharge treatment of polymeric surfaces to increase adhesion to that surface. The glow discharge (i.e., corona type discharge) in the vacuum reduces the yellowing typically resulting from corona discharge treatment by 75 to 80%. The surfaces are heated to a temperature below the glass transition temperature or melting point during glow discharge treatment.
U.S. Pat. No. 3,081,485 describes a process for heating and softening polymeric materials using electron-beam irradiation so that further mechanical treatment such as stretching and coating can be carried out. The energy densities used (e.g., column 2, line 15) are about two orders of magnitude higher than the energy densities used in the present invention. The energy levels described in U.S. 3,081,485 would cause ablation. The authors note on column 2, lines 26 ff. that small traces of irradiated material are evaporated during irradiation. Although the patent describes surface heating, the immediate depth of e-beam penetration (see column 3) appears to be greater than 150 microns. This form of energy would have equal effects on the bulk polymer and would not cause only surface modifications.
U.S. Pat. No. 4,631,155 describes the surface modification of polymers by subjecting the surface to at least one pulse of intense electromagnetic radiation. The surface polymer is disoriented during the relatively long exposure to radiation. Disorientation is indicative of an amorphous surface. Very thick amorphous layers appear to be formed as indicated by the chloroform test described in column 5.